30 FLUIDITY AND PLASTICITY 0 being the coefficient of sliding friction which is the reciprocal of the coefficient of slipping. From the experiments of Piotrowski upon the oscillations of a hollow, polished metal sphere, suspended bifilarly and filled with the viscous liquid, Helmholtz deduced a value for X of 0.23534 for water, but it is worth noting that he deduced a value of the viscosity which was about 40 per cent greater than that obtained by the efflux method. From some efflux experi- ments of Girard (1815) using copper tubes, Helmholtz deduced the value X = 0.03984. More recently Brodman (1892) has experimented with concentric metal spheres and coaxial cylin- ders, the space between being filled with the viscous substance. He thought that he found evidence of slipping. Slipping can be best understood in cases where a liquid does not wet the surface, as is true of mercury moving over a glass surface. If we consider a horizontal glass surface A, Fig. 6, as being moved tangentially toward the right over a surface E, FIG between which there is a thin layer of mercury C, then we can imagine that the mercury is separated from the glass on either side by thin films B and D of some other medium, usually air. Points in a surface at right angles to the above indicated by abed may at a later time occupy the relative positions a'Vc'd or if the films B and D are more viscous than the mercury the section may be better represented by a"6"c"d. But from Eq. (1) dv oc